This invention relates to silicon wafer electroplating and quantitative analysis of electroplating bath components. More specifically, it relates to analysis of electroplating bath constituents during integrated circuit fabrication. Even more specifically, the invention pertains to a particular monitoring and feedback system used for analysis and control of electroplating bath formulations and plating hardware.
Improved integrated circuit fabrication processes continue to necessitate more complex and demanding control of process parameters to ensure wafer uniformity and quality. Electroplating is a good example. Electroplating for integrated circuit fabrication is typically performed in a series of plating steps, with each having a particular hardware configuration and specific plating bath formulation. Often bath formulations include metal salts, acids, and organic additives. More than ever, it is critical to monitor plating bath electrolyte constituents and maintain bath formulations within a specific range of parameters to ensure the desired outcome and quality of a particular plating process.
Conventional methods of assaying bath constituents commonly employ cyclic voltammetric stripping (CVS) or other forms of Faradaic electroanalysis, which have limitations in specificity and sensitivity. For example, voltammetric analyses suffer from lack of detection capability for compounds and ions that are not electrochemically active over the range of potentials used. Additionally, voltammetric analyses are sensitivity-limited by matrix effects (convoluted electrochemical interactions due to the response of breakdown products).
High-pressure liquid chromatography (HPLC) has been proposed as a method to monitor plating bath constituents by Taylor et al. xe2x80x9cElectroplating Bath Control for Copper Interconnects,xe2x80x9d Solid State Technology, vol. 4, issue Nov. 11, 1998. In this article, the authors describe using HPLC to separate electrolyte species. Although HPLC techniques have improved dramatically over the past decade, this type of analysis has limitations with regard to plating bath composition. While organic additives such as accelerators, suppressors, and levelers are well suited for chromatographic separation, some important primary bath species, ions, metal salts, and acids are not. Analysis of purified bath components via chromatography can provide valuable information about organic plating bath electrolyte components, but only provides a partial picture of the plating environment.
Another problem associated with conventional plating bath analysis is time, or more specifically turnaround. Although analysis techniques have improved to include shorter analysis time frames, the time necessary for conventional analyses as compared to the time frame of possible change in a plating bath composition can be inadequate. Presently, concentrations of most chemicals in plating baths are measured by removing a sample from the bath and performing an analysis in a remote lab. Although these xe2x80x9coff-linexe2x80x9d measurements made in a separate lab are cost effective and reliable, the turnaround is often unacceptable for monitoring and controlling production equipment. Under such conditions, data regarding composition change obtained from plating bath analysis is rendered useless because the data may no longer reflect the actual bath formulation. This can be particularly problematic when such data is used to adjust bath component stoichiometries, i.e. the stoichiometry imbalance noted in the analysis can be compounded by addition of bath components based on inaccurate data.
An improved approach toward monitoring electrolyte composition is xe2x80x9con-linexe2x80x9d monitoring; that is, using a system that is integral to plating production hardware and is continually supplied with electrolyte sample for time efficient regular feedback to the plating system. Existing on-line monitoring systems for plating baths rely on titration of bath samples or cyclic voltammetry.
An example of an xe2x80x9con-linexe2x80x9d analyzer that uses cyclic voltammetry is the QUALI-LINE AC-1000, from ECI Technology of East Rutherford, N.J. This system has a relatively small footprint, but voltammetric methods suffer the drawbacks as described above. A more elegant approach is utilized by Technic, of Providence, R.I. with their RTA (real time analyzer) system. The RTA uses a probe that is immersed directly into a plating bath electrolyte. Although this system is very simple, and a good monitoring tool, the data obtained from cyclic voltammetry methods is not as accurate or reliable as desired for modem production plating environments.
Systems utilizing xe2x80x9con-linexe2x80x9d titration methods also have drawbacks. First, each titration requires one or more chemical reactants that are used only once with the sample being analyzed. These chemicals must be replenished. Second, detection of an endpoint for a titration usually requires an electrode that must be frequently calibrated. Third, such systems have large footprints, due to the syringe assemblies and reservoirs supplying the assemblies. Finally, titrations produce waste, which results in disposal issues.
Another alternative for on-line monitoring is ion chromatography. Besides having large waste streams, this method uses relatively expensive equipment and is of questionable reliability.
What is needed therefore is improved technology for on-line analysis and control of electroplating bath formulations during electroplating and electroplating processes during integrated circuit fabrication.
The present invention provides methods and apparatus for analysis and monitoring of electrolyte bath composition. Based on analysis results, the invention controls electrolyte bath composition and plating hardware. Thus, the invention provides control of electroplating processes based on plating bath composition data. The invention accomplishes this by incorporating accurate bath component analysis data into a feedback control mechanism for electroplating. Bath electrolyte is treated and analyzed in a flow-through system in order to identify plating bath component concentrations and based on the results, the plating bath formulation and plating process are controlled.
One aspect of the invention pertains to methods for monitoring and controlling an electroplating process. These methods may be characterized by the following sequence: (a) obtaining a sample of electrolyte, comprising an acid, a metal salt, and one or more organic components, from the electroplating process; (b) removing an organic fraction of the sample of electrolyte to give a substantially organic-free electrolyte sample; (c) determining the density of the substantially organic-free electrolyte sample; (d) determining at least one of the conductivity and the light absorption of the substantially organic-free electrolyte sample; (e) comparing at least one of the conductivity and the light absorption measurement of the substantially organic-free electrolyte sample with the density in order to determine a concentration value for each of the metal salt and the acid; and (f) adjusting conditions of the electroplating process in response to a comparison of the concentration value for each of the metal salt and the acid, with an associated target value. Methods of the invention can monitor plating bath chemistries xe2x80x9con-line,xe2x80x9d that is, during the plating process in real time.
In these methods, the sample of electrolyte is obtained directly from a plating cell of the electroplating process, from a separate sampling vessel of the electroplating process, or from a central plating chemistry vessel.
Methods of the invention find particular use in the context of copper electroplating in a damascene scenario. In damascene copper electroplating, typically copper sulfate, sulfuric acid systems are used. Organic agents are often added to impart leveling, suppressing, or accelerating elements to the plating environment. As well, other inorganic additives may be added such as chloride ion, in the form of hydrochloric acid.
In the latter case, additionally such methods would include determining a chloride ion concentration (preferably after the metal salt and acid concentration determinations), and adjusting the plating process accordingly with respect to a comparison of the chloride ion concentration with an associated target value.
In a preferred embodiment, removing an organic fraction of the sample of electrolyte typically includes a filtration of the electrolyte through a charcoal medium, molecular sieves or other agent specific for removing only organic species. In one embodiment, the used filter agent (typically in a cartridge or module format) is exchanged for new periodically. In an alternative embodiment, the organic fraction is removed from the on-line system, stripped from the filter agent, and analyzed by HPLC. Results from this analysis are also used as a basis for adjusting electroplating conditions based on comparison with target values. Thus, after HPLC analysis of the organic fraction, an adjustment of the electroplating process with respect to a comparison of at least one concentration of an organic bath constituent, obtained from the HPLC analysis, with a target concentration value for the organic bath constituent is made.
Adjusting conditions of the electroplating process comprises adjusting electroplating apparatus hardware. Preferably, this is done through addition of chemical stocks to a central electroplating bath chemistry vessel. Based on data from comparing analysis results to target values, chemical feedstock valves are opened and chemicals metered into a central bath to adjust plating bath chemistry. After analysis, the electrolyte samples are returned to the central electroplating bath chemistry vessel. Alternatively, adjusting conditions of the electroplating process comprises manipulating other electroplating apparatus hardware or functions, such as electrical current flow to a plating cell, adjusting a field shaping apparatus, adjusting a voltage level, adjusting a wafer handling apparatus, adjusting a relative orientation of an electrode with a counter electrode, and the like.
Other embodiments of the invention relate to apparatus for performing the method of the invention. Such apparatus comprising: (a) a device for sampling electrolyte from the electroplating process, wherein the electrolyte comprises an acid, a metal salt, and one or more organic components; (b) a module for removing an organic fraction from the electrolyte to give a substantially organic-free electrolyte sample; (c) a densimeter for determining a density of the substantially organic-free electrolyte sample; (d) a module for determining at least one of conductivity and light absorption for the substantially organic-free electrolyte sample; and (e) an associated logic for using at least one of the conductivity and the light absorption in the substantially organic-free electrolyte sample and the density measurement in order to determine a concentration value for each of the acid and the metal salt and controlling the electroplating process based on comparison of the concentration value for each of the metal salt and the acid, with an associated target value.
The device for sampling electrolyte can collect electrolyte directly from a plating cell of the electroplating process, from a separate sampling vessel of the electroplating process, or from a central plating chemistry vessel. In one embodiment the device for sampling electrolyte is a pump. Preferably, the pump delivers electrolyte at between about 1 and 20 ml/minute through the analysis system.
The module for removing an organic fraction from the electrolyte typically uses a charcoal medium as an organic adsorbent, however, molecular sieves or other adsorbent specific for removing only organic species can be used. In one embodiment, the module for removing an organic fraction from the electrolyte isolates the organic fraction for delivery to and subsequent HPLC analysis in, an HPLC module. Delivery of the isolated organic fraction to the HPLC module is done through standard plumbing and valves well known to those skilled in the art.
Once filtered, the substantially organic free electrolyte is pumped through the system to a densimeter. The densimeter used for the invention can be from a commercial source as long as a density measurement for the substantially organic-free electrolyte sample is made to within an accuracy of 0.0001 g/cm3.
After a density value for the electrolyte is determined, either the conductivity or the light absorption (or both) is determined. Apparatus for making the conductivity measurement and light absorption measurement preferably can determine a concentration value for each of the metal salt and the acid used in the electrolyte to within an accuracy of 0.1 g/L. The light absorption (absorptivity, extinction coefficient) is measured at a particular wavelength associated with determining concentration values most accurately. These components can be combined in a single module for determining at least one of the conductivity and the light absorption. Alternatively, either a spectrophotometer or conductivity cell would suffice to perform the method. In any case, flow-through systems are preferred.
At this point the associated logic compares at least one of conductivity and light absorption for at least one of the metal salt and the acid in the substantially organic-free electrolyte sample to the density of the substantially organic-free electrolyte sample in order to determine a concentration value for each of the metal salt and the acid. Based on a comparison of the concentration values for each of the metal salt and the acid with associated target values, the logic controls the electroplating process via manipulation of plating hardware.
The electrolyte can be returned to its plating hardware source at this time via a return line, or alternatively the electrolyte may travel through an additional apparatus in the on-line system and then returned. The alternative additional apparatus is a module for determining a chloride ion concentration measurement from the substantially organic-free electrolyte sample. If this apparatus is used, the chloride ion concentration measurement is also used as a basis for controlling the electroplating process by the associated logic. The chloride ion concentration measurement involves electrochemical oxidation of chloride ion to chlorine gas. Electrochemical cells to perform this analysis are common in the art.
Another aspect of this invention pertains to the logic associated with using plating species concentration data for feedback control of an electroplating process. Preferably data from an analysis is stored in a memory device. Then the data is compared to a data set of known target values for optimum plating performance. The comparison comprises determining whether the data from the on-line analysis falls within a specified tolerance of a target value. From the comparison, the logic determines commands for controlling the electroplating process. As mentioned, the invention finds particular use in the context of copper electroplating. Copper electroplating during damascene processing is becoming increasingly important and complex. The logic of the invention provides an efficient method of monitoring and controlling plating bath chemistry and hardware during electroplating. This allows for improvement in throughput and wafer uniformity.
Yet another aspect of this invention pertains to apparatus for controlling an electroplating process. Preferably, the control element comes in the form of commands sent to plating hardware by the logic as a result of a comparison of data from on-line analysis to target values. The associated logic of the apparatus controls plating hardware through adjustment, for example valves for introducing plating bath constituents and formulations, electric field shaping apparatus, current flow, voltage levels, wafer handling apparatus, and electrode movement apparatus. In many cases, individual components of the apparatus can be purchased commercially. Their configuration and programming constitute novelty in this case. The associated logic may be implemented in any suitable manner. Often it will be implemented in computer hardware and associated software for controlling the operation of the computer.